The present invention relates to vertical cavity surface emitting lasers and more particularly to the control of transverse laser modes using a photonic band gap structure having a light aperture.
The optical radiation emitted from lasers may have different spatial distributions. If a laser emits optical radiation that contains only the fundamental transverse mode, the radiation is a narrow laser beam with a Gaussian-distributed cross-section, most intense in the centre and less intense at the edges. In contrast, laser radiation having a higher transversal mode displays bright and dark spots across a cross-section of the radiation. The transverse electromagnetic modes are conventionally designated as TEM00, TEM01, TEM10, TEM11, etc. where TEM00 is the fundamental transverse mode and the others are higher transverse modes.
Radiation with higher transverse modes is normally undesirable because it is difficult to couple such radiation into optical fibres and to focus it for free-space beam forming. In addition, higher transverse mode radiation travels at somewhat slower speed in an optical fibre than the fundamental transverse mode radiation, thereby creating mode dispersion, i.e., broadening of an optical pulse as it travels in an optical fibre.
Vertical Cavity Surface Emitting Lasers (VCSELs) are semiconductor lasers having an optical cavity formed by mirrors which are parallel to a substrate on which the laser is formed. Thus, the optical cavity of a VCSEL is perpendicular to the substrate; and optical radiation is emitted from the VCSEL in a direction normal to the substrate. VCSELs are typically layered structures where the cavity mirrors are formed as stacks of distributed Bragg reflectors (DBR) around an active semiconductor layer.
VCSELs have many advantages over conventional edge emitting semiconductor lasers. For example, VCSELs can be made extremely small; VCSELs can easily be made into arrays that contain a large number of VCSELs; VCSELs can be tested at an early stage during manufacturing (on-wafer testing) which is an important parameter in the cheap production of VCSELs; VCSELs can be subject to very fast modulations compared to lasers in the same price-range.
The DBRs are either grown epitaxially with the rest of the structure, or deposited at a later stage. In the first case the mirrors are made from semiconducting material, whereas in the latter case the mirrors are made from dielectrics. The gain medium of the VCSEL is formed by providing an electrical current to the active layer. Typically, a small, micrometer sized current aperture is fabricated near the active layer to define a transverse extent of the gain region. When the mirrors are made from semiconducting material, the current injection to the active layer can happen through the mirrors by properly doping the semiconducting material. Dielectric mirrors cannot conduct current, and a lateral charge injection to the active layer is provided by electrical contacts.
The current aperture controls both the transverse extent of the gain region and the transverse mode lasing of the laser. A critical VCSEL design issue is related to the current aperture which laterally concentrates the injected carriers to provide large enough gain to overcome cavity losses and achieve lasing. The transverse dimension of the current aperture also determines the amount of power which can efficiently be coupled to lasing modes and thereby ultimately determines the obtainable output power of the laser. It is often necessary to have a current aperture which is large in relation to the transverse dimensions of the transverse modes in order to obtain a reasonable power output. As a result, the VCSEL lases in several transversal modes already at medium powers of a few mW. A typical circular shaped VCSEL with a current aperture diameter greater than 10 xcexcm, emits TEM00 mode radiation only at low currents. At higher currents, lasing in the higher transverse modes sets in.
U.S. Pat. No. 5,317,587 relates to a method of manufacturing VCSELs. The method uses dielectric current confinement in addition to transparent metal contacts and a mesa-shaped area to separately control the.injected current distribution and the optical mode.
Also, VCSELs typically emit radiation having uncontrolled directions of polarisation. In many applications (e.g. magneto-optical disks, optical communication applications etc.), lasers having predetermined directions of polarisation are highly desirable. Further, adjacent VCSELs in a VCSEL array have a tendency to couple with each other in an uncontrolled manner. In some instances, this results in unwanted beam cross sections.
U.S. Pat. No. 5,412,680 provides a method for controlling the polarisation and the lasing mode of VCSELs by using strained semiconductor layers with a preferred direction of conductivity.
JP 10 284 806 and JP 11 186 657 provide a VCSEL with a two-dimensional photonic band gap (PBG) structure in a plane parallel with the cavity mirrors. The PBG structure restricts spontaneous emission from the gain region so as to decrease cavity losses.
The article Enhanced coupling to vertical radiation using a two-dimensional photonic crystal in a semiconductor light-emitting diode by Erchak et al. (Applied Physics Letters, 78, 563, 2001), describes the use of PBG structures in the surface of light emitting diodes to enhance the light extraction efficiency up to six times.
W. D. Zhou et al., Electronics Letters, Vol. 36, no. 28, 1541 (2000) describes the experimental work with a surface emitting optical device. The device is made from the GaAs/AlGaAs system and consist of a bottom distributed Bragg mirror of n-type, an undoped lambda cavity with two embedded InGaAs quantum wells for optical gain, and a top p-type layer. The structure has been structured with a regular array of holes penetrating the top p-type layer, the lambda cavity with the embedded gain material, and a part of the bottom Bragg-mirror. The etching depth is in total 0.8 microns. The regular structure is intended to have a photonic band gap effect and have a defect defined by leaving out one of the holes. The structure is lasing on a mode defined by the defect by observing light emission concentrated in a region including the defect. The maximum output power from the device is reported to be 14.4 xcexcW.
In the described structure it is necessary to etch through the top and the gain region to get sufficient overlap with the field. The requirement of etching through the gain material restricts the total area of gain material within the defect area and reduces the resulting output power. The authors achieve 14.4 xcexcW. At least two orders of magnitude more power are needed to be useful in practice.
Separating the current confinement and the mode control can result in VCSEL with considerably increased power output and better beam properties, e.g. in the form of single mode operation. In the prior art, such separation has been performed by simply introducing large losses for higher order modes e.g. by providing a lower reflectivity for higher order modes as compared to the fundamental mode. However, since energy can be coupled between modes, this results in large energy losses leading to a low efficiency and low power output. The present invention provides a separation of the confinement of the gain region and the mode control, without the disadvantages of the prior art.
In a first aspect, the present invention provides a VCSEL comprising:
a semiconductor material layer having a gain region adapted to generate light and to emit the generated light,
first and second at least substantially parallel mirrors forming a laser cavity comprising the gain region and at least one spacer layer being positioned between the gain region and the first and/or the second mirror, at least one of the mirrors being partially transparent to the generated light so as to allow the light generated in the gain region to be emitted through said at least one mirror, the laser cavity and the gain region supporting at least one transverse electromagnetic mode for the generated light, and
a photonic band gap region formed within or adjacent to the first and/or the second mirror, or within one of the at least one spacer layer(s), the photonic band gap region being positioned at least substantially parallel to the first and second mirror, the photonic band gap region having a predetermined periodicity which substantially prevents the generated light from propagating in said region, the photonic band gap region further defining a light aperture without the predetermined periodicity, so as to allow the generated light to propagate through said light aperture, the dimensions of the photonic band gap region and the light aperture being adapted to at least partly control an efficiency of laser action in each transverse electromagnetic mode, the dimension of the photonic band gap region in a direction being substantially normal to the first and second mirror being significantly smaller than the overall dimension of the vertical cavity surface emitting laser in said direction.
Thus, according to the present invention, the current confinement and the mode control is separated by providing, in or adjacent to a VCSEL cavity, a PBG region having a light aperture controlling the lasing modes. It is an advantage of the present invention, that it controls the lasing in modes by suppressing or preventing laser action by providing regions into which transverse components of the radiation can not propagate. Thereby, there will be little or no laser action in modes extending substantially into these regions. Hence, the method does not suppress lasing by introducing severe losses for those modes, rather, the modes are made forbidden. Taking another point of view, the higher order modes are restricted in size whereby they obtain a much lower modal gain than the fundamental modesxe2x80x94hence, lasing in the fundamental modes is promoted compared to lasing in higher order modes. Radiation incident normally to the PBG region, within the PBG region or within the light aperture, will not be affected by the PBG region and hence will be reflected by the mirror as usual, without experiencing losses due to the PBG region.
Thus, the present invention provides sufficient power to allow the VCSEL to be of practical use because the full gain region is used. This is a great advantage of the present invention. In the structure described in W. D. Zhou et al., Electronics Letters, Vol. 36, no. 28, 1541 (2000), on the other hand, only a small part of the gain region is used, resulting in a reduced power to the extend that the output may not be used for practical purposes. A spacer layer may be positioned between the gain region and only one of the mirrors. Alternatively, one spacer layer may be positioned between the gain region and the first mirror, while another spacer layer is positioned between the gain region and the second mirror. Alternatively, two or more spacer layers (e.g. manufactured from various materials) may be positioned between the gain region and the first and/or the second mirror, or one or more of the spacer layers may be constituted by a number of layers, e.g. of various material.
By positioning the PBG region within or adjacent to the first and/or the second mirror, or within one of the spacer layer(s), it is ensured that the PBG region does not extend through the whole of the gain region. This is a great advantage since this ensures that the full gain region is available for providing the sufficient output power from the VCSEL.
Preferably, the PBG region is positioned in such a way that it does not intersect the gain region at all.
In the present application, the terms, transverse mode and mode refers to transverse electromagnetic modes conventionally designated as TEM00, TEM01, TEM10, TEM11, etc., unless otherwise stated.
Photonic band gap effects are well documented both experimentally and theoretically. They rely on a periodic modulation of an optical constant such as the material permittivity in one, two, or three dimensions by forming a one, two, or three-dimensional periodic microstructuring in the material. The microstructuring can either result in a modulated refractive index (etching of air holes) as well as in a modulation of the absorption coefficient by deposition of e.g. circular or differently shaped metal areas.
According to a second aspect, the present invention provides a polarised VCSEL according to the first aspect, but wherein the light aperture is elongated and has a dimension xcex1 along a first axis and a dimension xcex2 less than xcex1 along a second axis perpendicular to the first axis, and wherein the dimensions of the photonic band gap region and the elongated light aperture are adapted to at least partly control an efficiency of laser action in each transverse electromagnetic mode and to suppress or prevent laser action in transverse electric modes which are not polarised at least substantially parallel to the first axis.
According to a third aspect, the present invention provides a VCSEL according to the first aspect, but wherein the scattering centres are elongated along a first axis, and wherein the dimensions of the photonic band gap region and the light aperture are adapted to at least partly control an efficiency of laser action in each transverse electromagnetic mode, and wherein the elongation of the scattering centres is adapted to suppress or prevent laser action in transverse electric modes which are not polarised at least substantially parallel to the first axis.
The elongation of the scattering centres breaks a symmetry of the PBG region whereby an electric field polarised parallel with the first axis experiences a smaller scattering cross section from the scattering centres, than an electric field polarised perpendicular to the first axis. By experiencing a smaller scattering cross section from the scattering centres, lasing in a mode which is polarised parallel with the first axis will be suppressed less than lasing in a mode which is polarised perpendicular to the first axis.
Preferably, the elongated light aperture in a VCSEL according to the second aspect of the present invention has a two fold symmetry.
Preferably, the light aperture in a VCSEL according to the first or third aspect of the present invention has a three- or more-fold symmetry. Typically, the light aperture will be shaped as a regular polygon.
In the following, some preferred features of a VCSEL laser according to the first, second and/or third aspect of the present invention are described.
The laser cavity may support a plurality of transverse modes. In the present application, a transverse electromagnetic mode can be regarded as a distribution of wave-vectors k describing the propagation of the electromagnetic field. Any of the wave-vectors k may be projected onto a plane which is normal to the extension of the cavity, such as a plane parallel to the PBG region. The projection of k onto such as plane will be designated the transverse component kT of the wave-vector k, the term transverse referring to the extent of the cavity rather than to k.
The dimensions of the photonic band gap region and the light aperture preferably control the laser action for each transverse electromagnetic mode by controlling a ratio between transverse components kT lying within the light aperture and transverse components kT lying within the photonic band gap region. Thereby, the ratio between transverse components kT lying within the light aperture and transverse components kT lying within the PBG region determines a suppression or prevention of laser action in transverse electromagnetic modes so as to suppress or prevent laser action in modes having a low ratio in relation to modes having a high ratio. Preferably, the dimensions of the photonic band gap region and the light aperture are adapted to suppress or prevent laser action in higher order transverse electromagnetic modes in relation to laser action in a fundamental transverse electromagnetic mode of the cavity (TEM00).
If the VCSEL is electrically driven, it may further comprise current supply means for supplying an electric current to a region of the semiconductor material for defining the gain region. Alternatively, if the VCSEL is optically pumped, it may comprise means for supplying optical pumping to a region defining the gain region. In both cases, the gain region may be characterised in that it, in a plane at least substantially parallel to the first and second mirror, has a substantial overlap with two or more transverse electromagnetic modes of the cavity.
Thus, the separation of the current confinement and the mode control renders possible a much larger gain region while having mode control, such as while having single mode operation. Since the mode control is provided by the PBG region and the light aperture, the gain region may energetically couple to several transverse modes without deterioration of the single mode operation. According to the present invention, the energy coupled to other modes is not lost since the PBG region does not absorb radiation as such, rather, the energy eventually couples back to the fundamental lasing mode.
Typically, the first and second parallel mirrors are distributed Bragg reflectors (DBRs), one of which forms an output coupler by allowing a fraction of impinging radiation to be transmitted. The mode control according to the present invention does not restrict the PBG to the output coupling mirror. The light aperture still serves as an aperture for the light since it constitutes an allowed region for the transverse components of the wave-vector of the modes.
In a fourth aspect, the present invention provides a phased array of VCSELs comprising:
a semiconductor material body having a gain region adapted to generate light and to emit the generated light,
first and second parallel mirrors forming a laser cavity comprising the gain region and at least one spacer layer being positioned between the gain region and the first and/or the second mirror, at least one of the mirrors being partially transparent to the generated light so as to allow the light generated in the gain region to be emitted through said at least one mirror, and
a photonic band gap region formed within or adjacent to the first and/or the second mirror, or within one of the at least one spacer layer(s), the photonic band gap region being at least substantially parallel to the first and second mirror, the photonic band gap region having a predetermined periodicity which substantially prevents the generated light from propagating in said region, the photonic band gap region further defining two or more separated light apertures positioned within a region defined by a projection of the gain region onto a plane being adjacent to or within the first or the second mirror, the two or more light apertures being regions without the predetermined periodicity forming two or more coupled laser resonators, the dimensions of the photonic band gap region and the two or more light apertures being adapted to at least partly control an efficiency of laser action in transverse electromagnetic modes in each laser resonator.
The dimension of the PBG region in a direction being substantially normal to the first and second mirror may be significantly smaller than the overall dimension of the phased array of vertical cavity surface emitting lasers in said direction, thereby ensuring that the PBG region does not extend through the whole of the cavity, in particular not through the entire gain region.
Preferably, the PBG region is positioned in such a way that it does not intersect the gain region at all.
The dimensions and positions of the light apertures may be adapted to focus the emitted light at a distance from the laser. The apertures may take various shapes determined so as to optimise the focus of the far field. The two or more light apertures may also be similar to the light apertures according to the first or third aspect of the present invention.
In a fifth aspect, the present invention provides a method of manufacturing a VCSEL with control of transverse electromagnetic mode, the method comprising the steps of:
providing a substrate,
forming a first parallel stack of distributed Bragg reflectors on the substrate,
forming an active layer and a spacer layer on the first parallel stack,
forming a second parallel stack of distributed Bragg reflectors on the active layer and the spacer layer,
forming a photonic band gap region within or adjacent to the first and/or the second parallel stack, or within the spacer layer, by forming a periodic modulation in the permittivity of one or more layers of the first and/or the second parallel stack, or the spacer layer, said photonic band gap region being formed so as to delimit a light aperture for controlling the transverse electromagnetic mode, said light aperture being formed by a region in the photonic band gap region without said periodic modulation of the permittivity.
Preferably, the PBG region is formed and positioned in such a way that it does not intersect the gain region.
Preferably, the periodic modulation of the permittivity is formed by forming a periodic lattice of holes in the first and/or the second parallel stack. Alternatively, the periodic modulation of the permittivity may be formed by depositing semiconductor material or metallic pads in a periodic lattice on the second stack.
Depending of the desired properties of the VCSEL, the light aperture may have a three or more fold symmetry, such as by being shaped as a regular polygon. Alternatively, in order to control the polarisation of the radiation, the light aperture may have a two fold symmetry and be elongated along a first axis defining a polarisation axis for the modes. In this case, the light aperture typically has the overall shape of a rectangle, a trapezium, or an ellipse.
Preferably, the method comprises the steps of:
forming a current aperture for defining a gain region in the active layer, and
forming charge injection means for providing current to the active layer.
Alternatively, the first parallel stack may be formed by materials which are at least substantially transparent for radiation of wavelength xcexp, the active layer in this case at least substantially absorbing radiation of wavelength xcexp, so as to allow for optical pumping of the VCSEL.
In a sixth aspect the present invention provides a method of manufacturing an array of coupled VCSELs with individual control of transverse electromagnetic modes, the method comprising the steps according to the fifth aspect, wherein the step of forming the photonic band gap region comprises the step of forming the photonic band gap region so as to delimit two or more light apertures.
The first, second, third, fourth, fifth, and sixth aspect may each be combined with any of the other aspects.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.